GeogCS.as_cartopy_projection handling non-Earth planets (#4497)

* GeogCS passes itself as a globe to ccrs.PlateCarree

* Test that fails before change

* Bug or feature?

* Update as_projection test to use .srs

* Fix failing test

* Bug fix in test

* Capitalise Earth to bump tests

* Fix test_project to map between equal-sized planets

* Fix test

Author: wjbenfold

docs/src/whatsnew/latest.rst

@@ -153,6 +153,9 @@ This document explains the changes made to Iris for this release
    instead of one latitude and one longitude.
    (:issue:`4460`, :pull:`4470`)
 
+#. `@wjbenfold`_ stopped :meth:`iris.coord_systems.GeogCS.as_cartopy_projection`
+   from assuming the globe to be the Earth (:issue:`4408`, :pull:`4497`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/coord_systems.py

@@ -297,7 +297,10 @@ def as_cartopy_crs(self):
         return ccrs.Geodetic(self.as_cartopy_globe())
 
     def as_cartopy_projection(self):
-        return ccrs.PlateCarree()
+        return ccrs.PlateCarree(
+            central_longitude=self.longitude_of_prime_meridian,
+            globe=self.as_cartopy_globe(),
+        )
 
     def as_cartopy_globe(self):
         # Explicitly set `ellipse` to None as a workaround for

lib/iris/tests/test_coordsystem.py

@@ -188,6 +188,29 @@ def test_as_cartopy_globe(self):
         self.assertEqual(res, expected)
 
 
+class Test_GeogCS_as_cartopy_projection(tests.IrisTest):
+    def test_as_cartopy_projection(self):
+        geogcs_args = {
+            "semi_major_axis": 6543210,
+            "semi_minor_axis": 6500000,
+            "longitude_of_prime_meridian": 30,
+        }
+        cs = GeogCS(**geogcs_args)
+        res = cs.as_cartopy_projection()
+
+        globe = ccrs.Globe(
+            semimajor_axis=geogcs_args["semi_major_axis"],
+            semiminor_axis=geogcs_args["semi_minor_axis"],
+            ellipse=None,
+        )
+        expected = ccrs.PlateCarree(
+            globe=globe,
+            central_longitude=geogcs_args["longitude_of_prime_meridian"],
+        )
+
+        self.assertEqual(res, expected)
+
+
 class Test_GeogCS_as_cartopy_crs(tests.IrisTest):
     def test_as_cartopy_crs(self):
         cs = GeogCS(6543210, 6500000)

lib/iris/tests/test_mapping.py

@@ -211,7 +211,10 @@ def test_grid(self):
 
     def test_default_projection_and_extent(self):
         self.assertEqual(
-            iplt.default_projection(self.cube), ccrs.PlateCarree()
+            iplt.default_projection(self.cube),
+            ccrs.PlateCarree(
+                globe=self.cube.coord_system("CoordSystem").as_cartopy_globe()
+            ),
         )
         np_testing.assert_array_almost_equal(
             iplt.default_projection_extent(self.cube),

lib/iris/tests/unit/analysis/cartography/test_project.py

@@ -52,7 +52,7 @@ def setUp(self):
             1,
         )
 
-        self.tcs = iris.coord_systems.GeogCS(6000000)
+        self.tcs = iris.coord_systems.GeogCS(6371229)
 
     def test_is_iris_coord_system(self):
         res, _ = project(self.cube, self.tcs)